1. Field of the Invention
The present invention relates to a motor speed control apparatus where a motor immediately starts to rotate upon receipt of a starting signal.
2. Description of Prior Art
FIG. 3 shows a circuit diagram of one of the conventional motor speed control apparatus and also the conventional motor drive apparatus. In FIG. 3, first, the conventional motor drive apparatus 1 will now be described. A voltage (+B) is applied to each of Hall sensors 2 to 4 for detecting the rotation speed of the motor, and signals corresponding to impedance changes of these Hall sensors 2 to 4 are respectively amplified and then input into a logic circuit 8. Thereafter, from this logic circuit 8 to amplifiers 9 to 11, periodical signals corresponding to the rotation speed of the motor are sequentially supplied. To these amplifiers 9 to 11, speed control signals are supplied from the motor speed control apparatus 20. Impedances of PNP transistors 12 to 14 are lowered in accordance with the amplitudes of the speed control signals only while the signals from the amplifiers 9 to 11 are being supplied thereto, and accordingly, motor drive currents corresponding to shifts of the motor rotation speed from a motor setting speed flow through motor coils 15 to 17. It should be noted that resistors 18 to 20 function as biasing resistors by which the PNP transistors 12 to 14 are not conducted while the signals are not output from the amplifiers 9 to 11.
Referring now to a waveform shown in FIG. 4, the motor speed control apparatus 20 will be described. A sine wave, as illustrated in FIG. 4a, is supplied to a waveform shaping circuit 22 in response to the motor rotation speed from a frequency signal generating circuit (e.g., a frequency generator etc. ) coupled to the motor. A rectangular wave, as shown in FIG. 4b, is supplied from this waveform shaping circuit 22 to a sample-and-hold pulse generator 23. As a result, the sample-and-hold pulse generator 23 supplies a sampling pulse, as illustrated in FIG. 4c, to a sawtooth wave generator 24 simultaneously upon receipt of the rising edge of the rectangular wave, and supplies a holding pulse, as represent in FIG. 4d, to a holding circuit 25 at a slightly earlier time instance than the rising edge of the rectangular wave. In addition, the sawtooth wave generator 24 outputs a sawtooth wave, as indicated in FIG. 4e, the voltage of which increases at a constant time-constant in synchronism with the sampling pulse, and thereafter supplies the sawtooth wave to the holding circuit 25. Then, the holding circuit 25 will hold a substantially, or near peak voltage of the sawtooth wave by means of a capacitor 26 in response to a holding pulse. Since the discharging time constant of this capacitor 26 is great, the voltage across the capacitor 26 is not practically changed at the sampling pulse period. Then, the voltage across this capacitor 26 is applied as a signal voltage illustrated in FIG. 4f from the holding circuit 25 to a plus (positive) input terminal of a differential amplifier circuit 27. A speed setting voltage "S" applied from a speed setting circuit 28 is applied to a minus (negative) input terminal of the differential amplifier circuit 27 so as to set the rotation speed of the motor. Then, a difference between the signal voltage output from the holding circuit 25 and the speed setting voltage is amplified in the differential amplifier circuit 27 and thereafter applied as a speed control signal to the motor drive circuit 1 via a buffer circuit 29.
When the rotation speed of the motor is slow, the period of the sampling pulse becomes long, and the signal voltage output from the holding circuit 25 becomes also high. Accordingly, when the rotation speed of the motor is slower than the setting speed, the speed control signal of the higher voltage is supplied to the motor drive circuit 1 so as to control the motor to be rotated at a more higher speed. It should be noted that the terminal of the capacitor 26 at the side of the holding circuit 25 is grounded via a switch 30, which forms an operation control circuit.
FIG. 5 is a detailed circuit diagram of the above-described conventional operation control circuit. In FIG. 5, the terminal of the capacitor 26 at the side of the holding circuit 25 is grounded via a first NPN transistor 31. A base of this NPN transistor 31 is connected via a resistor 32 to the power supply (+B) and grounded via a second NPN transistor 33. A base of the second NPN transistor 33 is connected in series with the power supply (+B) via a third NPN transistor 34 and a resistor 35.
When the signal supplied to the base of the third NPN transistor 34 is "L" as illustrated in FIG. 6, the second NPN transistor 33 is not conducted whereas the first NPN transistor 31 is conducted, so that the capacitor 26 is brought into a shortcircuit condition and thus, no speed control signal is output. When the signal (G) is inverted to "H" as the starting signal, the third transistor 33 is conducted and the first NPN transistor 31 becomes non-conductive. Then, the capacitor 26 begins to be charged, as illustrated in FIG. 6 (F), by the higher voltage applied from the sawtooth wave generator 24. When the voltage across the capacitor 26 exceeds the speed setting voltage "S", the speed control signal is output and therefore the motor driving operation starts.
As previously described, when the voltage across the capacitor 26 exceeds the speed setting voltage "S" after the charging operation of the capacitor 26 starts in response to the starting signal, the motor driving operation will start. However, since the rotation speed of the motor is slow due to just after the motor starting, the period of the sampling pulse becomes long and furthermore, the capacitor 26 is charged so that the voltage across the capacitor 26 is coincident with the speed setting voltage "S" after a predetermined time lapse. As a result, a time period "t.sub.1 " for which the motor reaches a stable speed, is required longer than the charging time of the capacitor 26. In general, this time period must be considerably longer than the charging time.
To solve the above-described conventional problem involved in the prior art motor speed control apparatus, an object of the present invention is to provide a motor speed control apparatus for simultaneously starting the motor upon receipt of the starting signal.